Preplanned series

I don't find it in bad form; I mean, it's best for each story to have a resolution of SOME kind, even if they're only defeating a small part of the problem and not the main problem itself. And it's always good for a series story to help set up what occurs in later stories, even if this isn't made clear until one has read the entire series. "Oh! THAT'S why they did that in Part 1! I didn't even notice that until now!"

I write series stories myself and even though each one has its own villain and there isn't really an overarching theme of one SUPREME bad guy controlling all of it, each story, while standing alone, does help build up to the next. For example in my current WIP there are a few small references to what will later become the main threats in the next story--but they're very vague and referred to only in passing, as they're not the main threat of THIS story, and who knows if a reader will return for the sequel. There's foreshadowing, but it's not terribly obvious until the reader returns for the next story. Likewise, characters' actions in previous stories help build up to what happens in later stories.

My point being, overarching themes are fine, especially in a series, though you should work hard to make the individual stories stand well on their own. The defeat of the minor bad guy you mention sounds like a good start. It's the first story of a series, but there's a plot with a beginning, middle, and end--a resolution. And there's a teaser to interest readers in the next story. Sounds good to me.

That's just my opinion though.

I think it is okay so long as each story can be read individually without a feeling of 'oh something is missing' over and over.

Not everyone will know that it is a series. This happened to me with Diana Gabladon's Outlander series. I read the second book Dragonfly in Amber first. 95 % of the book stood alone as an indivdual story but I did get the feeling in certain parts that I had missed something.

Just make sure there are too many loose ends hanging around.